Real Estate Assets (Details) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2017 USD ($) property	Mar. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) property	Sep. 30, 2016 USD ($)
Dispositions [Abstract]
Gains on disposition of property	$ 19,849		$ 3,902	$ 25,181	$ 14,160
Impairments [Abstract]
Impairments of real estate assets	$ 1,445		$ 0	$ 1,445	$ 0
2017 Dispositions [Member]
Dispositions [Abstract]
Number of buildings sold | property	12			12
Purchase price of real estate	$ 78,000	$ 13,000
Disposition closing credits excluded	2,500	1,200
Gains on disposition of property	19,800	$ 5,300
2017 Impairments [Member]
Impairments [Abstract]
Impairments of real estate assets	$ 1,400